During the manufacture of airplanes, finished smooth and polished surfaces become scratched so that they are not acceptable to the purchaser. Such surfaces are aircraft windows, windshields, and polished aluminum clad panels which form the general exterior surface of the aircraft. Scratch depth must be determined to make sure that it is not too deep and that the scratch can be removed without weakening the material beyond acceptable limits.
Prior attempts to determine scratch depths included using mechanical measuring techniques, comparators, and subjective visual examinations. Visual examinations were made by an inspector, who would look at a "standard" scratch and then compare it to a scratch under inspection on an aircraft part. Prior optical instruments used for measuring scratches were designed for use on metallic and other opaque, matte surface finishes. In order to measure a glossy or specular surface, the surface had to be sprayed with a white matte coating. This technique would often fill shallow scratches and would cause considerable errors in the measurement of deeper scratches.
A survey of the market showed no small portable instrument which could measure scratches to 0.0005" on any material with a polished to medium fine surface finish. A search of the patent literature disclosed patents including a series of lenses, a light source, mirrors and a slot or knife edge as an image producing device that is used for comparative readings. For example U.S. Pat. No. 2,216,003 discloses a microscope, FIG. 2, having a prism which has a reflecting hypotenuse covering half of the exit pupil. Laterally of the microscope, there is a diaphragm having an illuminated slit, the diaphragm being disposed opposite the prism so that the ray path from the slit of the diaphragm through the prism to the microscope objective corresponds to the ray path from the objective to a plane containing the mark of the glass plate.
U.S. Pat. No. 2,379,263 discloses apparatus for measuring the flatness of a surface of a glass plate. The apparatus includes a standard plate having a polished surface and means to support a plate to be tested adjacent to and in spaced parallel relationship to the polished surface of the standard plate. Light passes through one plate to strike the surface of the other, the light source being disposed to project a laterally elongated band of light obliquely upon one plate and through the latter upon the other plate. A diffusing screen is adapted to receive images of the bands reflected from the plate to be tested and the standard plate to permit accurate comparison of the images.
U.S. Pat. No. 3,619,578 discloses apparatus for indicating the surface depression of softwood veneer. Light from a microscope illuminator is directed between a straightedge and a surface of the veneer passing in sliding contact underneath the straightedge. A diode detects light reflected above a predetermined intensity from underneath the straightedge as an indication that the depth of the depression is above a minimum. An electronic counter counts the number of such depressions per unit length of veneer.
U.S. Pat. No. 3,396,627 discloses a device for measuring the roughness of a surface. It includes at least one planar target .member having one or more targets or groups of spaced lines thereon and being carried by support means, which may rest against the surface to be measured, and is adapted to hold the target member adjacent that surface in a plane at an acute angle relative to the surface, and siding means also carried by the support means and establishing a fixed line of sight of the image of each target or group of lines formed by reflection thereof onto the surface to be measured at an angle of incidence greater than 45.degree..
The following list of patents disclose optical instruments of general interest:
U.S. Pat. No. 2,446,628, Brown PA1 U.S. Pat. No. 3,314,328, Boettcher PA1 U.S. Pat. No. 3,667,846, Nater et al PA1 U.S. Pat. No. 3,734,626, Roberts et al PA1 U.S. Pat. No. 3,804,521, Sprague PA1 U.S. Pat. No. 3,857,637, Obenreder PA1 U.S. Pat. No. 3,922,093, Dandliker et al PA1 U.S. Pat. No. 4,017,188, Sawatari PA1 U.S. Pat. No. 4,030,830, Holly PA1 U.S. Pat. No. 4,145,140, Fujii PA1 U.S. Pat. No. 4,155,098, Roach et al PA1 U.S. Pat. No. 4,158,507, Himmel PA1 U.S. Pat. No. 4,202,630, Suzuki et al PA1 U.S. Pat. No. 4,363,118, Roach et al PA1 U.S. Pat. No. 4,364,663, Gardner et al